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Marantz 4300 dim bulb test

(*steve*)

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You can create a larger image and post it to an external image hosting site, then post a link to it here.

You should be able to safely power up the amplifier with parts disconnected/removed.

It makes almost no sense to keep the output transistors connected if the amplifier board is removed though.

One or both of the output transistors have probably shorted out.

I'm very concerned that there is a significant change in current as the volume control is advanced. I am almost certain that you are seeing this with no input connected. The confounding detail is that none of the output transistors were getting warm, even though they appear die in spectacular fashion.

What is the rating of your bulb?

can you confirm that if you disconnect amplifier boards that even one channel operates properly?

I'm going to check where that resistor is now...
 

(*steve*)

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OK, looking at that schematic again.

You should replace both H003 and H004.

You should check that H713 is not shorted from collector to emitter, and possibly consider replacement.

You should probably replace R745 (certainly if it's resistance is no longer close to 100R)

Then you'll need to do the bias adjustments again (if they are possible).

Then you need to read some voltages.

The ones I would like to see are:

Base of H003 and H004 (or emitter of H714 and H713)
Collector of H714 and H713
Emitter and collector of H737 and H707

If you find a board that is working, compare these to the one that isn't/wasn't.

These measurements should be done with no signal, the dummy load connected, and at various volume levels (if that makes a difference) AND with your light bulb in series with the power.
 

mrmodify

Feb 13, 2010
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The bulb on the dim bulb tester is a 120volt 100 watt soft white bulb.

I cannot operate with just one amplifier board. It acts the same, with no main spkr button volume can be turned up. Main spkr. in, relays start cycling with volume increase.

H003 & H004 will have to be replaced. R745 is still reading 100 ohms but am going to replace. Checked H713 & H714 to be good. H 717, H 719 & H721 are good. H711 is gone. Checked H727(called a Variactor), H729 (diode) these are good also. Transistors were removed and checked with a transistor checker.

Was last schematic ok?

What is H711 purpose. I had to replace it in the beginning. NTE's cross is a NTE 85. I researched the original (2sc458(B) which come up obsolete. NTE's cross also shows a NTE 123AP also. Was wondering if the original cross is wrong. I used a NTE85. This transistor is mounted to the heatsink with H713, H714, & H727(variactor).

It will probably will be a week or two before outputs will come in.:confused:
 

(*steve*)

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H711 is there as part of the biasing. The adjustment of R739 that you make to get 12mV between (9) and (11) is setting the quiescent current through H003 and H004.

If you look at the circuit, this is the voltage across R761 which is (I presume) a high power low resistance centre tapped resistor. Incidentally, if you can measure the resistance of this, it would be interesting. It's not marked on the diagram, but I would suspect it is probably very low in value.

It's kinda tricky to see why H711 is failing. How has it failed? i.e. what is shorted/open?
 

mrmodify

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Morning Steve, H711 is shorted between C & E. other terminals are normal.

H003 is shorted C to B. E to B and E to C are open.

H004 is completely shorted.

R761 is .2 ohm +.2 ohm 10% 5W by manual. Ohm Meter shows .5 ohms. It is made like one big ceramic resistor with 4 leads. 2 leads are tied together.

Attached picture is of the amp, The square white resistors on bottom are R761 on left &762 on right. H713 & H714 are on the heatsinks to the left.
H727 is mounted on heatsink with leads attached. H 711 is on the left under heatsink. If you look close you can see it is removed for now.
 

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(*steve*)

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I'll have to think about that for a while.

The question is whether H711 died first, or as a consequence of H003 and/or H004.

My thoughts are that when H711 is turned ON, the bias to H003/4 is reduced, so it seems unlikely that it failing CE would do anything but reduce the drive to H003/4

Still thinking...
 

mrmodify

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Currently taking voltage measurements on board that has not failed. found that H711 pin outs are marked wrong on this board. Later tonight I will install a transistor in H711 and install board with the good outputs and record voltage readings.
 

(*steve*)

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Had you (or someone else) replaced the H711 previously?

Is it possible tat you replaced it with an incorrect orientation?

I am thinking that when you replace the H003/4 that you initially place some resistors in series with the emitters (something like 10R 10W) which should serve to protect the transistors but allow output at low volume.

I may need to think about that a little more too...
 

mrmodify

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Hello Steve, Yes H711 has been changed before and I installed it backwards.
I have found the PCboard is marked wrong. The base connection on H711 is not the same on both boards. I installed the new NTE85 with the base on the same pad as the cathode of the variactor.


I am going to check the H711 & H712 transistor and confirm orientation on all the boards. This would not be the first thing I found reversed in this receiver. I liked your quote earlier. Someone dicked around with this unit.

Don't know if I mentioned it but I keep the power switch on the receiver on and control the power with a light switch on the dim bulb tester. I cannot turn switch on and have the speaker button depressed.

If I do relays will cycle. If I cut speaker button off without killing power and let power settle for a minute I can press speaker button and all is fine.
 
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Resqueline

Jul 31, 2009
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I don't know if I can contribute more than Steve has already done, but I just want add that an oscilloscope could be useful to see if the amp' is oscillating.
I came across a Kenwood receiver once that would oscillate (& burn out the outputs) if the treble was turned down low.
A capacitor pair was not installed (as marked) on the PCB. I experimented forth a minimum value and all was well.
I did replace a handful of output transistors before I put a 'scope to it - and discovered what was going on.
 

(*steve*)

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Yeah, it's not all that uncommon to have to replace transistors more than once because you think you've found the problem but haven't. :(

That's especially true of smaller transistors where limiting the supply current doesn't prevent you from letting the smoke out.

Don't feel too bad about killing another pair of output transistors. You might want to buy more than one pair. That's generally what you need to do to find the fault and make them superfluous! :D

We also can't completely rule out problems in the preamp. If it is oscillating then you'd get increasing power as the volume is turned up, but it's odd that you can turn the power up slowly...

DC coupled amplifiers like this one can be a little (lot) tricky as a DC offset fault in any part of the amplifier can affect later stages. You can't as easily troubleshoot each stage individually.
 

mrmodify

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Attached voltage readings that I recorded last night. I posted them last night but think I'm getting dyslexic in my old age so I had to make corrections. :eek:

I didn't see anything out of spec to bad. Some voltages are low but for the most part ok? :cool:

Going to get away from it tonight, tomorrow going to check for parts installed incorrectly by me or other. :eek:

Think I will take my scope and check on the preamp out jacks and see what happens. :)
 

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